In printing using the electrophotographic process, the electrostatic latent image formed on a photoreceptor is developed by a toner to form a toner image (visible image), then the toner image is transferred onto paper or another transfer material. The transferred toner image is heated and pressed by a heat pressure roll to be fixed on the transfer medium.
As the toner, in the past, a toner obtained by the pulverizing method where a binder resin obtained by polymerizing a polymerizable monomer is melt kneaded with a colorant etc., then pulverized and classified (pulverized toner) has generally been used. However, with the pulverizing method, in particular when producing a fine particle size toner, there are the problems of a drop in the yield, consumption of a large amount of energy for the pulverizing, etc. To solve these, a toner obtained by the polymerization method (polymerized toner) has been proposed.
As the process of production of a polymerized toner, there are the suspension polymerization method, emulsion polymerization method, etc. For example, with the suspension polymerization method, first the polymerizable monomer, the colorant, and, in accordance with need, a charge control agent, molecular weight modifier, or other additives are mixed to obtain a polymerizable monomer composition, then this is dispersed in an aqueous medium having a dispersion stabilizer. Next, the aqueous medium in which the polymerizable monomer composition is dispersed is given a high shear by a high speed stirrer etc. to form droplets of the polymerizable monomer composition. After this, these are polymerized in the presence of a polymerization initiator, filtered, washed, and dried to obtain dried colored polymer particles. Further, these colored polymer particles are mixed with a carrier and/or fine inorganic particles or other external additives to obtain a polymerized toner.
However, some of said additives obstruct polymerization. Further, unreacted polymerizable monomer easily remains in the colored polymer particles. Furthermore, in addition to the polymerizable monomer, compounds produced as a byproduct of the polymerization initiator, the molecular weight modifier, and other volatile matter sometimes remain.
A toner with these volatile matter remaining in it easily falls in high temperature shelf stability. If using this toner for printing, the heating at the time of fixing causes the residual low molecular weight components to volatilize resulting in an offensive odor and degrading the surrounding environment in some cases. Further, in the case of continuous printing, in particular continuous printing under a high temperature and high humidity, there is the problem that the durability easily drops.
To deal with this problem, as a method for removing the polymerizable monomer remaining in the toner after polymerization, the process of production of a toner treating a dispersion containing colored polymer particles after suspension polymerization by reduced pressure stripping, then drying it has been proposed. A toner using t-butylperoxy-2-ethylhexanoate (molecular weight 216) as the polymerization initiator is disclosed (Japanese Patent Publication (A) No. 2000-321809). Further, as a method of removing the polymerizable monomer remaining in the toner after polymerization, the process of production treating the toner by reduced pressure stripping under specific agitation conditions has been proposed. A toner using t-butylperoxy-isobutyrate (made by NOF Corporation, product name “Perbutyl IB”, molecular weight 160, purity 74%) as the polymerization initiator has also been disclosed (Japanese Patent Publication (A) No. 2001-117272).
However, in these toners, the amount of unreacted polymerizable monomer is reduced and the amount of polymerizable monomer remaining in the toner is reduced, but this cannot be said to be sufficient. Compounds produced as a byproduct of the polymerization initiator etc. remained in large amounts and the obtained toner did not have sufficient durability of printing, in particular durability of printing under a high temperature and high humidity. Further, if excessively treating the toner to remove more of the remaining low molecular weight components, the colored polymer particles agglomerated during the treatment and the obtained toner easily dropped in print durability, so there were limits.
As a method of reducing the amount of decomposed product of the polymerization initiator and residual monomer (polymerizable monomer) remaining in the toner after polymerization, a process of production of a toner resin using as the polymerization initiator at least one substance selected from the group of the aromatic peroxide dicinnamoyl peroxide and C4 to C8 tertiary alkylperoxy cinnamate (the molecular weight of the one with the smallest molecular weight among these being 220) has been proposed (Japanese Patent Publication (A) No. 2000-3076). If using a polymerization initiator according to this method, the polymerizable monomer remaining in the obtained toner, the compounds produced as a byproduct of the polymerization initiator, etc. remain in large amounts. The obtained toner was not sufficient in durability of printing, in particular durability of printing under a high temperature and high humidity.
Further, to deal with the above problem, as toners limiting the volatile matter remaining in the toners, the toners mentioned below have been disclosed. Japanese Patent Publication (A) No. 2004-287426 discloses a toner containing volatile components comprised of carnauba wax and 4 to 60 ppm of ketones obtained by the method of dissolving a binder resin, colorant, carnauba wax, and other components in an organic solvent, emulsifying the solution in an aqueous medium, then distilling off the solvent and causing agglomeration to produce colored particles. Further, Japanese Patent Publication (A) No. 2001-255699 discloses a toner containing a styrene-based binder resin polymerized using an organic peroxide, having a content of carbonic acid derived from the organic peroxides of less than 2,000 ppm, and having a content of styrene of less than 300 ppm.
Further, Japanese Patent Publication (A) No. 2000-321809 discloses a toner having less than 70 ppm of residual polymerizable monomer and having a fluidity of 55% or more.
However, the toners described in these publications are not sufficient in durability of printing, in particular durability of printing under high temperature, high humidity conditions. Further, they had the problems of easy blocking at the time of storage at a high temperature, that is, a drop in the high temperature shelf stability.